The present invention relates to an objective optical system for an optical pickup apparatus capable of conducting recording and/or reproducing of information compatibly for optical information recording media each being different from others, an optical pickup apparatus, a driving apparatus for an optical information recording medium and a condenser lens.
In recent years, there has been an advance of a trend to a shorter wavelength of a laser light source used as a light source for reproducing of information recorded on an optical disc or recording of information on an optical disc, and there has been started a practice of a laser light source with a wavelength 400-420 nm, such as, for example, a violet semiconductor laser and a blue SHG laser that converts a wavelength of an infrared semiconductor laser by the use of a second harmonic. If these violet semiconductor lasers are used, information of 15-20 GB can be recorded on an optical disc having a diameter 12 cm when an objective lens having the same numerical aperture (NA) as in DVD (digital versatile disc) is used, and information of 23-25 GB can be recorded on an optical disc having a diameter 12 cm when NA of the objective lens is enhanced to 0.85. In the present specification, an optical disc and a magneto-optical disc which use the violet laser light source are generically called “high density disc” hereafter. In the case of some high density optical discs using the objective lens having NA 0.85, a protective layer is designed to be thinner than that in DVD (0.1 mm for 0.6 mm in DVD) to reduce an amount of comatic aberration caused by skewing, having the same numerical aperture (NA) as in DVD (digital versatile disc) is used.
(Patent Document 1) Japanese Patent Publication TOKKAIHEI No. 10-123410, (Patent Document 2) Japanese Patent Publication TOKKAI No. 2001-236680.
First of all, as the first problem, there is a demand for downsizing of an optical pickup apparatus capable of conducting recording and/or reproducing of information for a high density optical disc, in the case of a use, for example, for a notebook computer. In this case, it is preferable to use an objective lens having a small diameter for downsizing of the optical pickup apparatus. On the other hand, an objective lens complying with high NA for high density recording and/or reproducing is disclosed in Patent Document 1, and this objective lens has two combined lenses each having positive paraxial power, and therefore, a height of a point for light to pass through on an optical surface on the optical disc side grows lower, thus, the working distance of the high NA objective lens which tends to be small originally becomes smaller, which is unsuitable for an objective lens having a small diameter.
Next, the second problem will be explained. In general, ability to conduct recording and/or reproducing of information properly for high density optical discs is not enough as a value of an optical disc player and/or recorder product. Presently, when considering the reality that DVD and CD (compact disc) on which various types of information are recorded are on the market, mere ability to conduct recording and/or reproducing of information for high density optical discs is not enough, and ability to conduct equally recording and/or reproducing of information properly also for DVD and CD owned by a user enhances a commercial value as an optical disc player and/or recorder for high density optical disc. From the background of this kind, the optical pickup apparatus mounted on an optical disc player and/or recorder for high density optical disc is required to have capability to conduct recording and/or reproducing of information properly while keeping compatibility for any of high density optical disc and DVD, and further of CD.
As a method to conduct recording and/or reproducing of information properly while keeping compatibility for any of high density optical disc and DVD, and further of CD, there is considered a method to switch an optical system for high density optical disc and an optical system for DVD and CD, selectively depending on recording density of the optical disc for recording and/or reproducing of information. This method, however, is unsuitable for downsizing and cost increase is caused, because a plurality of optical systems are needed.
Contrary to this, if a light source unit wherein a light-emitting section for a high density optical disc, a light-emitting section for DVD and further a light-emitting section for CD are united collectively is used, a light-converging optical system can be used commonly, resulting in an advantage of downsizing of an optical pickup apparatus and cost reduction. However, when using the light source unit wherein plural light sources are united solidly, at least one light-emitting point is shifted from the optical axis of the light-converging optical system, and an image height is caused. For solving the image height, therefore, there are proposed various methods employing a prism and a diffracting element. For example, for the light source unit provided with two light-emitting sections each having a different wavelength, there is a method wherein an optical axis of a light-emitting section on one side is made to be a reference, and a light-emitting section on the other side is aligned with that reference. However, when light fluxes each having a different wavelength are emitted from the common light source unit to the coupling lens (including a collimator lens) existing in the optical path, there is a problem that a focal length fluctuates, and a magnification of the light flux entering the objective optical element fluctuates, depending on a wavelength. When light fluxes each having a different wavelength are emitted from the common light-emitting unit to the beam shaping element, there is a problem that astigmatism is caused. In this case, it is considered that the coupling lens, the beam shaping element or the light source unit itself is moved in the optical axis direction for adjustment, which, however, causes a problem of cost increase resulted from a driving mechanism provided, and of an increase of a space for installation (a tendency to grow larger). In place of the driving system of this kind, it is considered to cope with the problem by means of an optical functional surface having wavelength-selectance such as a diffractive surface, which results in a problem that a quantity of light declines because it is difficult to keep the diffraction efficiency to be high (Patent Document 2).
Further, the third problem will be explained. One of the matters that demand special attention for manufacture of an objective lens composed of single lenses having numerical aperture on the image side NA of 0.85 for conducting recording and/or reproducing of high density information is that shift error in the optical axis direction between optical surfaces (hereinafter referred to as surface shift error) causes occurrence of coma to be great. This is caused by an angle of inclination of the aspheric surface (an angle formed by the optical axis and a normal line on the aspheric surface) at the outermost area within the effective diameter on the optical surface of the objective lens on the light source side, which grows greater when the numerical aperture NA becomes high. Further, in the design of the objective lens having high NA, the problem of a working distance needs to be considered sufficiently, in addition to the surface shift error stated above. In this case, the angle of inclination of the aspheric surface is in the tendency to become gentle as the refractive index is made greater. However, if the refractive index is made greater excessively, the lens turns out to be of a meniscus form that is convex toward the light source side. Therefore, on the optical surface on the optical disc side, the peripheral area in the effective diameter is more projected toward the optical disc side than the vertex on the surface is projected, which results in a problem that the working distance is shortened. If the refractive index is further made greater, the lens becomes to be in a meniscus form, and an effect to reduce coma caused by the surface shift error is lost, although an angle of inclination of the aspheric surface is made to be more gentle. Accordingly, when designing an objective lens having high NA, it is necessary to control occurrence of coma caused by surface shift errors, and to secure the working distance sufficiently, by selecting a material having the optimum refractive index within a range to avoid a meniscus form for the lens. A condition for the optimum refractive index varies depending on design conditions such as a lens thickness on the optical axis and a magnification. However, there is no example disclosing the condition for the optimum refractive index conforming to the design conditions.